Light-emitting diode (LED) devices, also referred to as electroluminescent (EL) devices, have numerous well-known advantages over other flat-panel display devices currently in the market place. Among these advantages are brightness of light emission, relatively wide viewing angle, and reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs) using backlighting. Such devices may be made with light-emitting layers comprising organic materials or inorganic materials such as quantum dots.
Applications of LED devices include active-matrix image displays, passive-matrix image displays, and area-lighting devices such as, for example, selective desktop lighting. Irrespective of the particular LED device configuration tailored to these broad fields of applications, all LEDs function on the same general principles. An electroluminescent (EL) medium structure is formed between two electrodes. At least one of the electrodes is light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the LED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge-carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The emitted light is directed towards an observer, or towards an object to be illuminated, through the light-transmissive electrode. If the light-transmissive electrode is between the substrate and the light-emissive elements of the LED device, the device is called a bottom-emitting LED device. Conversely, if the light-transmissive electrode is not between the substrate and the light-emissive elements, the device is referred to as a top-emitting LED device.
The EL medium structure can be formed of a stack of sublayers comprising organic materials that can include small-molecule layers and polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art, for example U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al., and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al. Alternatively, inorganic materials may be employed to form the EL medium structure, for example including core\shell quantum dots formed in a polycrystalline, semiconductor matrix, for example, as taught in pending U.S. application Ser. No. 11/683,479, by Kahen.
Because light is emitted through an electrode, it is important that the electrode through which light is emitted be sufficiently light transmissive to avoid absorbing the emitted light. Typical prior-art materials used for such electrodes include indium tin oxide and very thin layers of metal. However, the current carrying capacity of electrodes formed from these materials is limited, thereby limiting the amount of light that can be emitted.
In conventional integrated circuits, bus connections are provided over the substrate to provide power to circuitry in the integrated circuit. These buses are located directly on the substrate or on layers deposited on the substrate, for example on planarization layers. In complex circuits, multiple levels of bus lines are located over the substrate and separated by insulating layers of material. For example, OLED displays sold by the Eastman Kodak Company utilize multiple bus lines located on the substrate and on various planarization layers. However, these buses are not useful to provide power to the light-transmissive upper electrode in an OLED device because conventional photolithography techniques destroy the organic layers and a typically thin upper electrode employed in a top-emitting OLED device.
U.S. Patent Application Publication 2002/0011783 A1 proposes to solve this problem by the formation of auxiliary electrodes in contact with the upper or top electrode. The auxiliary electrode may be either above or below the upper electrode. The auxiliary electrode has greater thickness and conductivity thereby increasing the current carrying capacity of the upper electrode. However, this approach has difficulties in that it reduces the light-emitting area of the OLED device and is difficult to manufacture. In particular, if the auxiliary electrode is formed before the organic elements are deposited, forming a good electrical contact between the upper and auxiliary electrodes is difficult, because the organic materials will be deposited on the auxiliary electrode. Moreover, undesirable moisture can infiltrate through materials at the corners of the auxiliary electrode and the conformal deposition of an additional upper electrode protection and encapsulation layer is problematic. If the auxiliary electrode is deposited above the upper electrode, a patterned deposition process is relatively difficult and liable to destroy both the upper electrode and the organic layers beneath it.
A second prior-art method to address this problem is to use an auxiliary electrode, as proposed by U.S. Patent Application Publication 2001/0043046 A1 by Fukunaga et al. entitled “Luminescent Apparatus and Method of Manufacturing the Same.” However, this approach requires a complex multistep processing method and suffers from the above-described difficulties.
U.S. Patent Application Publication 2002/0158835 A1 by Kobayashi et al. entitled “Display Device and Method of Manufacturing the Same”, discloses the use of auxiliary wiring elements which are electrically connected to a light transmissive second or upper electrode of an active matrix type planar display device. The auxiliary wiring elements are formed in the same layer or on the same surface as first or lower electrodes, and the auxiliary wiring elements are electrically insulated from the first electrodes. However, Kobayashi et al. provide no drawings describing process steps used in a method of making the device. Moreover, the electrical connection disclosed by Kobayashi et al. is formed between partition walls. The construction of suitable partition walls adds complexity to the process, reduces yields, adds cost, and limits the resolution of the interconnections.
The use of lasers and other techniques to form patterns in integrated circuits is known. For example, U.S. Pat. No. 6,468,819, entitled “Method for Patterning Organic Thin Film Devices Using a Die”, describes the use of a die to form patterns and references the use of laser ablation to form patterns. U.S. Pat. No. 6,444,400, entitled “Method of Making an Electroconductive Pattern on a Support”, likewise describes ablation, including the use of infrared lasers. Other patents, for example U.S. Pat. No. 6,433,355 issued Aug. 13, 2002, entitled “Non-Degenerate Wide Bandgap Semiconductors as Injection Layers and/or Contact Electrodes for Organic Electroluminescent Devices”, also describe the use of laser ablation for patterning. However, none of these methods address problems with power distribution in a top-emitting LED device.
U.S. Pat. No. 6,995,035 entitled “Method of making a top-emitting OLED device having improved power distribution” by Cok and VanSlyke describes a method of making a top-emitting OLED device, includes providing over a substrate laterally spaced and optically opaque lower electrodes and upper electrode buses which are electrically insulated from the lower electrodes; depositing an organic EL medium structure over the lower electrodes and the upper electrode buses; selectively removing the organic EL medium structure over at least portions of the upper electrode buses to reveal at least upper surfaces of the upper electrode buses; and depositing a light transmissive upper electrode over the organic EL medium structure so that such upper electrode is in electrical contact with at least upper surfaces of the upper electrode buses. However, such a method tends to form particulate contamination when the organic EL medium structure is selectively removed. The particulate contamination may fall over the EL medium structure and inhibit current flow through the EL medium structure after a subsequent deposition of the light-transmissive upper electrode. This will cause unwanted dark spots.
There is a need therefore for an improved method and structure for providing enhanced power distribution to the transparent electrode of a top-emitting LED device.